|I'm assuming the IDE drive is connected to a mboard IDE header. |
If it's connected to a card in a slot, the led on the case may not blink or light up for it. The card often has pins to connect an led to.
It also could a problem with the data cable.........
It is common to un-intentionally damage IDE data cables, especially while removing them - the 80 wire ones are more likely to be damaged. What usually happens is the cable is ripped at either edge and the wires there are either damaged or severed, often right at a connector or under it's cable clamp there, where it's hard to see - if a wire is severed but it's ends are touching, the connection is intermittant, rather than being reliable.
Another common thing is for the data cable to be separated from the connector contacts a bit after you have removed a cable - there should be no gap between the data cable and the connector - if there is press the cable against the connector to eliminate the gap.
80 wire data cables are also easily damaged at either edge if the cable is sharply creased at a fold in the cable.
Try another data cable if in doubt.
Sometimes the led gets pulled out of it's mounting in the case front when you've been fiddling inside the case, especially if you removed the case front - the led may no longer be where it's supposed to be. That's happened to me several times.